1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine which includes a so-called lean NOx catalyst in an exhaust conduit of the engine.
2. Description of the Prior Art
Japanese Patent Publication Nos. HEI 1-130735 and HEI 1-135541 disclose an NOx catalyst constructed of zeolite carrying a metal selected from the group consisting of transition metals and noble metals and capable of reducing nitrogen oxides (NOx) under oxidizing gas conditions in the presence of hydrocarbons (HC). Such catalyst will be called a lean NOx catalyst hereinafter. The lean NOx catalyst needs HC to reduce NOx. Japanese Patent Publication No. SHO 63-283727 discloses a system wherein an HC supply apparatus is provided and HC is continuously supplied from the HC supply apparatus to a lean NOx catalyst when HC included in engine exhaust gas is insufficient, for example, when the exhaust gas temperature is high and the HC included in engine exhaust gas is directly oxidized into CO.sub.2 and H.sub.2 O.
However, there are problems with the prior art systems. More particularly, even if HC is supplied when the exhaust gas temperature is high, almost all of the supplied HC is directly oxidized into CO.sub.2 and H.sub.2 O and cannot be effectively used for reducing NOx. As a result, the NOx reduction rate of the lean NOx catalyst will not be increased as much as expected.
Further, if HC is continuously supplied, the merit (good fuel economy) of a lean burn engine will be lost. Furthermore, such continuous supply of HC will increase heat generation due to oxidation of HC at the lean NOx catalyst to raise the catalyst temperature. As a result, the lean NOx catalyst will be thermally degraded.